Porous materials have been widely used for daily requirements and modern industries from long ago because they can be utilized in important applications, such as filtering and purifications systems, acoustic and thermal insulation, building constructions, transportation, biomaterials, communications, aeronautical applications, etc. These special materials possess unique combinations of properties such as light-weight and excellent sound absorption due to the existence of a large number of pores that can lead to attenuation of sounds, high impact energy absorption arising from their large strains under relative low stresses, and high damping originating from the vibration of cell walls and the friction of cracks, as well as high gas permeability, etc.
According to the connections of pores, porous materials can be categorized as closed-cell and open-cell. In most cases, the applications such as filtration, separation, and sound or energy absorption require open-cell morphologies. Thus, porous metals with open-cell morphologies have wider applications in functional structures.
Many methods are currently recognized in the art for manufacturing metallic foams. According to one method, related to self-expanding foams, the liquid metal is mixed with a blowing agent which in turn generates gas bubbles throughout the metal matrix resulting in the foaming morphology, (US 2004/0079198 A1). In this method, it is difficult to get uniform foam structures due to inability to evolve blowing gas and disperse it throughout the matrix at optimum rate.
In order to avoid the non-uniform structure of produced foam, US 2010/0098968 A1 proposes a new fabrication method in which a metal foam structure is fabricated by filling the spaces around the readymade hollow metallic spheres with a metal matrix-forming material. Thus, the produced foam will have a symmetric morphology. The main difficulty in this technique is limited pore size range.
Manufacturing method of a metal foam in which a self-supporting, net-shaped porous preform is infiltrated by molten metal or impregnated with the matrix metal, wicking process, has been proposed in a number of patents. U.S. Pat. No. 5,679,041 A proposes a manufacturing technique in which a durable perform, composed of self-supporting fugitive polymeric particles without separate interparticle bonding, is filled by a molten metal. Prior to filling the preform with the metal, the polymer is evaporated giving a network of capillaries of the original polymeric foam morphology.
US 2008/314 738 discloses open-cell metal foam prepared by using a fugitive, open-cell, polymeric foam substrate consisting of a plurality of ligaments interconnected by nodes which together provide a three dimensional network of interstitial cells. The three dimensional network of the polymeric foam substrate is impregnated with a slurry of the filler particles suspended in aqueous solution media. The interstitial cells are filled with about 5% to 90% by volume particles. Thus, upon drying about 30% to 95% by volume void space generates between particles for subsequently molten filling. Producing, stable and durable preform using this method is quite difficult.
U.S. Pat. No. 3,694,325, relates to formation of a metal foam by electrodepositing a layer of the metal onto a fugitive foam substrate (polyurethane) which in turn is burned off, leaving a hollow metal network. This method can not be applied for the large dimension scale of products.